Like most new technologies, "storage virtualization" is a term that means different things to different people, and some of its definitions overlap with existing technologies and concepts. But most people use it to refer to the use of devices that sit somewhere on the network and bundle several storage units, potentially from different vendors, which they then present as a single resource. But while this concept seems appealing, it hasn't really taken off in practice -- except for in data migrations.

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The good news is that data migration is a major pain point for customers who are upgrading their storage hardware, and virtualization does a good job of easing the process. Traditionally, a company would have to take down the old drive, copy the data to the new one, and then bring the new one online. While that process is straightforward, the amount of data companies now store can make it very disruptive; larger drives may take a few days to copy over, during which they are unusable.

Virtualization lets companies migrate data to new drives, potentially without any downtime. You or your client's IT staff add the new drive as a subsystem to the virtualized storage and designate it as the new target for a given LUN instead of the old drive. The system will then begin copying all of the data from the old drive to the new one, and any I/O requests made in the process will be routed to the right drive in the process.

This use of virtualization is especially useful for storage VARs who are responsible not just for delivering the hardware, but for deploying it and carrying out the migration process. Some VARs have begun providing storage virtualization as a service for customers who have bought new storage systems. The VAR installs storage virtualization tools, performs the migration, and then takes away the virtualization layer and leaves the customer with the new hardware, fully migrated. While this approach does require some downtime for the installation and removal, it's a matter of an hour or two instead of several days, said Ray Lucchesi, president of Silverton Consulting in Broomfield, Colo.

In fact, some vendors are developing lightweight storage virtualization products designed specifically for this purpose, said George Crump, founder and senior analyst at consulting firm Storage Switzerland LLC. Large customers who have a lot of storage hardware, and thus more frequent purchases, may want to buy a system of their own, Lucchesi said.

Even if standalone appliances aren't flying off the shelves, storage virtualization is gaining traction as an internal management tool in some disk arrays. To computers and IT staff, these arrays appear pretty much like standard storage, and they don't require much knowledge of the underlying technology, Crump said. This lets storage vendors include advanced features like thin provisioning and data striping, which requires a layer of abstraction from a disk's logical functionality and its physical capacity. In fact, this may be the predominant use of storage virtualization -- both as a technology and as a term -- in three to five years, Crump said.

Pursuing the holy grail of storage virtualization

If your client does want to take storage virtualization to its fullest by trying to use it to consolidate and manage all of its storage devices, there are some caveats you should be aware of. The most striking is that backing out of virtualized storage isn't a trivial task. While storage virtualization products tend to be good for migrating disks into the system, many aren't as good at migrating them back out, Crump said; his advice to customers who want to transition away from virtualized storage is to treat the nonvirtualized system as a new storage purchase altogether. Some vendors are better than others at this reverse process, so unless your customer is 100% sure it wants to keep the system until the next hardware refresh, it's a good idea to make sure the product you're using can handle both directions of virtualization.

It's also important to understand the technology's drawbacks. Storage virtualization devices generally work either in-band or out-of-band. The in-band approach is simplest, but because every I/O request has to go through the virtualization appliance, this approach creates a bottleneck that can increase lag and introduce a point of failure. The out-of-band approach has less latency but requires an agent on any host that needs to access the storage devices. This can increase maintenance for companies that have many servers, and it's important to do your due diligence to reduce the risk of an update to the server's operating system being incompatible with the storage virtualization agent.

As with any virtualization technology, the ease of provisioning can be a double-edged sword. If your client uses storage virtualization extensively, it's important to help its management come up with a comprehensive provisioning plan so that the company doesn't end up with the equivalent of virtual server sprawl. And storage virtualization can also make disaster recovery more of a headache, since it's no longer clear which physical device is storing what information. Virtual storage vendors provide software to help discover what went where, but it's important to be aware of that extra step when a drive fails.

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